1. Field of the Invention
The present invention relates to a lens control apparatus to be preferably used in a video camera.
2. Description of the Related Art
Recently, video cameras or camcorders have become remarkably widespread, and many improvements have been made in performance, function, and operability thereof. Particularly, miniaturization thereof and increase in magnification of zooming are strongly demanded, and many attempts have been made to achieve them.
The reason why miniaturization of the video cameras is realized in these circumstances is that lenses of an internal focusing type which are small and capable of high-magnification zooming are adopted.
FIG. 1 schematically illustrates a configuration of a commonly used lens system of the internal focusing type.
Referring to FIG. 1, there are provided the first fixed lenses 101; the second lenses for varying magnification (hereinafter, referred to as a variator lens); a diaphragm 103, the third fixed lenses 104; the fourth lenses 105 (hereinafter, referred to as a focus lens) having both a focusing function and a so-called compensator (focus compensation) function of compensating for a shifting of a focal plane due to a magnification varying, and an image pick-up surface 106.
According to the lens system constructed as shown in FIG. 1, since the focus lens 105 has both the compensator function and focusing function, the position of the focus lens 105 for focusing on the image pick-up surface 106 varies with object distances even if focal lengths are equal. And, it is needless to say that the position of the focus lens 105 varies with the focal lengths even if the object distances are equal.
FIG. 2 is the plot of the position of the focus lens 105 for focusing on the image pick-up surface when the object distances are varied in each of the focal lengths. If a locus shown in FIG. 2 is selected in accordance with the object distance during magnification varying, and the focus lens 105 is shifted in accordance with the locus, a zooming without defocus becomes possible.
According to a lens system of a for-element focusing type, a compensator lens is provided separately from the focus lens with respect to the variator lens, and the variator lens and compensator lens are coupled by means of a mechanical cam ring. Therefore, when a knob for manual zooming is provided to vary the focal length manually, the cam ring follows the knob to rotate however fast the knob may be actuated, so that the variator lens and compensator lens shift along a groove of the cam ring. Thus, defocus is not caused by the zooming when the focus lens is in focus.
In a zoom control of the lens system of the internal focusing type having characteristics as described above, it is popular that a plurality of locus data shown in FIG. 2 are stored in a lens control microcomputer in one form or another, the locus of the focus lens is selected in accordance with the positions of the focus lens and variator lens, and the zooming is performed by tracing the selected locus.
Further, since the position of the focus lens with respect to the variator lens is read out from a memory device so as to be utilized for controlling the positions of the lenses, the position of each lens must be read out accurately to some extent. Particularly, as is also apparent from FIG. 2, the inclination of the locus of the focus lens varies every moment with the change of the focal length when the variator lens shifts with constant or nearly constant speed. This shows that the shifting speed and shifting direction of the focus lens change every moment. In other words, an actuator of the focus lens must perform accurate speed response from 1 Hz to several hundred Hz.
As an actuator which satisfies the above-described requirement, the use of a stepping motor in the focus lens of the internal-focusing lens system is becoming popular. Since the stepping motor rotates in complete synchronism with stepping pulses output from the lens control microcomputer or the like, and a stepping angle per pulse is constant, it is possible to obtain a high speed response and, stopping accuracy and position accuracy can be obtained.
In addition, the use of the stepping motor offers the following advantage. Since a rotation angle of the motor with respect to the number of stepping pulses is constant, the stepping pulse can be used as an incremental encoder, and there is no need to provide additionally a specific position encoder.
As described above, when the magnification varying is performed while maintaining in-focus with the use of the stepping motor, it is necessary to store the locus data of FIG. 2 in the lens control microcomputer or the like in one form or another (either the locus itself or a function having a variable of the lens position will do), read out the locus data in accordance with the position or the shifting speed of the variator lens, and then move the focus lens based on the data.
FIGS. 3A and 3B illustrate an example of the already proposed locus follow-up method.
FIG. 3B shows a memory table in the lens control microcomputer in which the locus data of FIG. 3A are stored. As apparent from FIG. 3B, shifting ranges of the variator lens and focus lens are split into a plurality of areas, and focus lens data a0, a1, . . . , b0, b1, . . . determined by the variator lens positions z0, z1, . . . and the object distance are stored in order. In FIG. 3B, v represents the variator lens position, n represents the object distance and each of the data Anv (n=0, 1 . . . m; v=0, 1 . . . s) are focus lens position data which are unitarily determined by the variator lens position and object distance.
In FIG. 3A, each of z0, z1, z2 . . . z6 represents the variator lens position; each of a0, a1, a2 . . . a6 and each of b0, b1, b2 . . . b6 represent typical loci of the focus lens stored in the lens control microcomputer. And, each of p0, p1, p2 . . . p6 represent the locus of the focus lens calculated from the above-described two loci. The locus is calculated by the following expression: EQU p(n+1)=.vertline.p(n)-a(n).vertline./ .vertline.b(n)-a(n).vertline..multidot..vertline.b(n+1)-a(n+1).vertline.+a (n+1) (1)
The expression (1) shows that when the focus lens is on p0, a ratio of a line segment b0-a0 divided internally by p0 is determined and a point which divides internally a line segment b1-a1 in accordance with the above ratio is taken as p1. A standard shifting speed of the focus lens for maintaining in-focus can be found from the position difference between p1 and p0, and the time involved in shifting of the variator lens from z0 to z1.
A case will now be described where there is no restriction such that the variator lens should stop only on the border having the stored typical locus data. FIG. 4 is a view for explaining an interpolation method of the variator lens position in which a part of FIG. 3A is extracted and the variator lens is at the voluntary position.
In FIG. 4, the vertical axis represents the focus lens position and the horizontal axis represents the variator lens position, respectively, and the typical locus positions (the focus lens position with respect to the variator lens position) stored in the lens control microcomputer are represented by a0, a1 . . . ak-1, ak . . . an and b0, b1 . . . bk-1, bk . . . bn according to the object positions when the variator lens positions are Z0, Z1 . . . Zk-1, Zk . . . Zn, respectively.
When the variator lens is on Zx which is not the zoom border and the focus lens position is px, ax and bx are determined by the following expressions: EQU ax=ak-(Zk-Zx)(ak-ak-1)/(Zk-Zk-1) (2) EQU bx=bk-(Zk-Zx)(bk-bk-1)/(Zk-Zk-1) (3)
That is, ax and bx can be determined by internally dividing one of the four stored typical locus data (ak, ak-1, bk, and bk-1 in FIG. 4) of the same object distance by the internal division ratio obtained from the present variator lens position and two zoom border positions (for example, Zk and Zk-1 of FIG. 4) which sandwich the present variator lens position. And, pk and pk-1 can be determined by internally dividing one of the four stored typical locus data (ak, ak-1, bk, and bk-1 in FIG. 4) of the same object distance by the internal division ratio obtained from ax, px and bx of expression (1). When zooming from a telephoto side to a wide view side, a shifting speed of the focus lens for maintaining in-focus can be found from the difference between the follow-up position pk of the focus lens and the present position px of the focus lens, and the time involved in shifting of the variator lens from Zx to Zk. When zooming from a wide view side to a telephoto side, the standard shifting speed of the focus lens for maintaining in-focus can be found from the difference between the follow-up position pk-1 of the focus lens and the present position px of the focus lens, and the time involved in shifting of the variator lens from Zx to Zk-1. The locus follow-up method as described above already has been proposed.
FIG. 5 is a flowchart showing a control of the above-described system which is usually processed in a lens control AF (automatic focusing) microcomputer. The processing is started from S1. A reset routine S2 resets RAM and various ports in the AF microcomputer. A communication routine S3 exchanges data of a zoom switch instructing the zooming and data of magnification varying, such as a variator lens position, with a system control microcomputer (hereinafter, referred to as a system controller). An AF processing routine S4 processes a sharpness signal of an AF evaluation signal to perform automatic focusing in accordance with a change in the evaluation signal. A zoom processing routine S5 is a routine for processing an operation of a compensator lens to maintain in-focus during the zooming. In this routine, a standard drive direction and a standard drive speed of the focus lens which traces the locus shown in FIG. 3 are calculated.
A drive direction/speed select routine S6 selects the drive directions and drive speeds of the variator lens and focus lens calculated in S4 and S5 in accordance with the automatic focusing and magnification varying. This routine prevents the lenses from being driven beyond the telephoto end, beyond the wide view end, beyond the closest end and beyond the infinity end which are specified on the programs so that the lenses do not butt against mechanical ends. S7 outputs a control signal to a motor driver in accordance with the data of the drive directions and drive directions of the variator lens and focus lens determined in the routine S6, so as to control drive/stop of the lenses. After completion of processing in S7, the procedure returns to the routine S3. A series of processing in FIG. 5 are performed in synchronization with a vertical-synchronizing signal (the processing in S3 waits for the next vertical-synchronizing signal to come). That is, in a video camera, since focus data for automatic focusing is detected in a field cycle (a cycle of the vertical-synchronizing signal), a flow of the control also synchronizes to the vertical-synchronizing signal and is repeatedly performed in the cycle.
However, because zoom speed has increased in recent years so the variator lens shifts, for example, from Z4 to Z6 in FIG. 3A during the vertical-synchronizing period. Thus, when the above-described operation is performed once during the vertical-synchronizing period, the focus lens shifts from p4 to p6' to defocus by p6'-p6, whereby the locus can not be traced exactly during the zooming. The term "vertical-synchronizing period" means a cycle of the vertical-synchronizing signal, i.e. a field period.